1. Field of the Invention
The present invention relates to the field of apparatuses and methods for imparting torsion to a tubular member. In the drilling industry such devices are called "tongs" and they are used to rotate drill pipe, tubing, and casing.
2. Description of the Prior Art
The applicant deems the following prior art material to this application and it is the closest prior art of which the applicant is aware: U.S. Pat. No. 3,892,140; and U.S. Pat. No. 3,906,820. The rotary drive apparatus of U.S. Pat. No. 3,892,140 employs a chain to impart torsion to a tubular member. It does not exhibit the unique features of the present invention nor is it capable of functioning with the efficiency and flexibility of the present invention. The subject of U.S. Pat. No. 3,906,820 is an apparatus and method for spinning pipe which does not secure a tubular member; which does not protect a tubular member; which does not protect a tubular member from elastic deformation; and which does not provide efficient screwing and unscrewing of a tubular member.
The prior art apparatuses employ cams with very flat curvatures to convert peripheral forces of a drive means into the requisite gripping or clamping force. In order to hold or to rotate a tubular member, these forces must be of a very high magnitude. This often results in the penetration of the blades of the device into the tubular member. Efforts to then release the blades result in a rotation in an undesired opposite direction. These large forces may also result in damage to the apparatus itself, such as undesired expansion or deformation.
The magnitude of the gripping and clamping forces can be reduced by employing cams with greater curvature rather than flat curvature cams. But the use of greater curvature cams has resulted in contact forces too low to initiate an unscrewing operation. Prior art apparatuses have overcome this disadvantage in an inefficient and makeshift way by accomplishing an unscrewing operation in at least two steps: (1) using flat curvature cams to initiate unscrewing and (2) refitting the apparatus with greater curvature cams that would finish the unscrewing operation without the undesired results of tubular member penetration or apparatus deformation.